There are two types of telecommunications networks that are of interest to the discussion of the invention, namely, circuit switched networks and asynchronous transfer mode ("ATM") networks.
1. Circuit Switched Networks
A circuit switched network may be used to provide voice communication paths between telephone subscribers. In the circuit switched network, a subscriber originating a call to another subscriber signals an end office, that is, an office of the telephone service provider, by taking a telephone handset off of its hook. After receiving a dial tone from the end office, the caller dials the digits of the telephone number of the subscriber with whom he wishes to speak. The end office then uses the dialed information to determine an efficient voice transmission path to the called subscriber through the circuit switched network.
The circuit switched network consists of transmission facilities, such as, for example, serial transmission lines or fiber optic cables, and switches that interconnect the transmission facilities. There are generally two types of switches in a circuit switched telecommunications network, namely, an end office switch and a toll/tandem, or trunk-side, switch. The end office switch attaches to subscriber equipment (telephone sets) on one side and interoffice trunks on the other side. The toll/tandem switch attaches to the interoffice trunks only. Interoffice trunks provide communication paths between two end offices, between end offices and toll/tandem switches, and between two toll/tandem switches.
Typically, digital transmission and digital switching technologies are used by the circuit switched network to achieve high-quality transmission of the voice signals. Before the voice signals are transmitted over the network from the end office switch, they are encoded in accordance with a pulse code modulation (PCM) scheme. With PCM, the voice signals are sampled 8,000 times per second (or once every 125 microseconds), and each sample is quantized to an 8-bit value. The transmission rate is thus 64 kb/s for a single channel.
The end office generally time division multiplexes the channels, to allow a number of calls to be serviced simultaneously over a single high-speed digital transmission facility. With time division multiplexing ("TDM"), PCM samples from each of the multiplexed channels are included in a TDM transmission frame that is then transmitted by the end office switch over the transmission facility. If the transmission facility is a T1 carrier, for example, the TDM transmission frames include 24 8-bit samples that represent 24 separate channels. Each channel is assigned a frame timeslot position in the transmission frame, and 20 one sample from each channel is included in each frame. The frames are transmitted once every 125 microseconds, or once per PCM sample period, with successive samples from the individual channels included in the corresponding timeslot positions in the succeeding frames.
The timeslot assignments on the switch's inbound and outbound transmission facilities are decoupled. Thus, for example, a call may be assigned timeslot position 17 in the transmission frames transmitted to the switch on an inbound TDM transmission facility, and assigned timeslot position 5 on the frames transmitted from the switch on an outbound TDM transmission facility. The channels are, of course, full duplex, and the timeslot assignments for the transmission frames that return to the call originator through the switch are generally the same as the timeslot assignments in the transmission frames that are transmitted through the switch from the call originator. That is, for a given transmission facility between two switches, the same timeslot position is used for both transmission directions of the call.
A switch in a circuit switched network that receives a transmission frame over an inbound TDM transmission facility must thus remove the PCM samples from the frame timeslots and transfer the samples to designated timeslots in one or more frames that are to be transmitted over one or more outbound TDM transmission facilities. The timeslot assignments are determined by the switch controller, as part of the connection establishment operations using inter-switch signaling mechanisms.
The switch architecture of interest includes a high-speed parallel bus that receives PCM samples from and transfers the samples to the inbound and outbound TDM transmission facilities that are attached to the switch. The switch also includes interface circuits, or cards, that transfer the PCM samples between the high-speed parallel bus and the TDM transmission facilities. Each interface card must have access to every line of the fines of the parallel bus during every PCM sample period. The interface card thus includes a high-speed driver for each line of the parallel bus. Examples of this type of architecture include the MultiVendor Integration Protocol (MVIP) and Signal Computing System Architecture (SCSA) bus standards.
The capacity of the switch is determined by the speed of the parallel bus and the number of lines on the bus. In order to avoid blocking at the switch, the parallel bus must have a bandwidth that is equal to the sum of the capacities of the inbound transmission facilities. To increase the capacity of the switch, the speed of the parallel bus must be increased and/or the number of lines in the bus must be increased.
The speed of the bus is limited by existing technology. The number of lines is limited essentially by the cost of the interface circuitry required to provide access to the bus. To minimize costs, two or more inbound TDM transmission facilities may be associated with a single interface card, and thus, share the interface circuitry. However, the cost of the interface cards necessarily increases as the speed of the parallel bus and/or the number of lines in the bus is increased. Accordingly, it is relatively expensive to increase the capacity of the switch, even with the shared use of the interface cards.
One way to increase the capacity of the switch without increasing the complexity of the interface cards is to use a multistage switch. The multistage switch combines the capacities of multiple lower-capacity switches, and thus, multiple high-speed parallel buses, to operate as a single high-capacity switching system. One example of a multistage switch uses a second switching stage in what is commonly referred to as a time-space-time switch architecture. The time-space-time switch architecture includes a center space-division switch matrix, commonly referred to as a time-multiplexed switch, which transfers the PCM samples from the timeslots on one set of multiple high-speed parallel buses to the timeslots on a second set of multiple high-speed parallel buses.
The space-division matrix, which is similar in operation to a cross-bar switch, is reconfigured once per TDM timeslot. The interface circuitry that connects the buses to the switch matrix ensures that the PCM samples are appropriately time multiplexed, that is, that the samples from the inbound transmission frames are placed in the appropriate timeslots of the outgoing transmission frames. The incoming and outgoing frames for the two transmission facilities are decoupled, in the sense that the PCM samples associated with a call may be assigned to different timeslot positions in the two frames.
A limitation of the space-division switch matrix is that the switch matrix must be custom designed for each multistage switch. If the capacity of the multistage switch is to be increased by adding more lower-capacity switches, a new switch matrix is required. Further, the lower-capacity switches must be physically present at the switch matrix, and thus, these switches cannot be distributed over the network.
A fiber ring may be used as the second stage of the multistage switch. However, this configuration has the same scaling problems as the single stage circuit switch. Specifically, in order to increase the overall capacity of this multistage switch beyond the capacity of a single ring, the system designer must add to the system one or more additional fiber rings and the relatively expensive interface circuitry that is required to connect the parallel buses to these rings.
2. ATM Networks
ATM technology is becoming more widely used in telecommunication networks that transfer data and video signals as well as voice signals. An ATM network consists of ATM switches, high-speed ATM links that interconnect the switches, and user-network access links that interconnect the users with the network.
In an ATM network, information is transported over the ATM link in fixed-length, 53-octet cells. Communications over the ATM network take place by the relaying of the fixed-length ATM cells over connection paths determined by a network routing procedure. The cells are thus routed over designated ATM links and through designated switches between a source edge and a destination edge of the connection path. Cells from several different connections may be multiplexed and transported over the same ATM link to a switch, in which case the switch demultiplexes the cells before it relays them to the appropriate outgoing links.
Each 53-octet ATM cell consists essentially of a 5-octet header that contains information necessary to relay the cell and a 48-octet cell payload that contains the user data. The cell header includes a Virtual Path Identifier (VPI) and Virtual Channel Identifier (VCI) value that identifies the connection. When a switch receives a cell, the switch uses the VPI/VCI value in the cell header to associate the cell with the appropriate path, and then sends the cell over the next ATM link on the path. The VPI/VCI value is actually associated with the link over which the cell traveled to the switch. The switch thus updates the VPI/VCI value before relaying the cell to the next switch over the designated link. The VPI/VCI values associated with each link on the connection path are determined by network signaling procedures when the connection is established.
The use of both a path identifier, VPI, and a channel identifier, VCI, as distinct components of the VPI/VCI value provides a two-level hierarchy of multiplexing and switching. Paths are multiplexed over an ATM link using the VPIs, and channels are multiplexed within the paths using the VCIs. A group of channels that are directed along the same ATM links between two points in a network can be routed by virtual path switching, to simplify the cell relay operations at intermediate switches.
The mapping of user data into ATM cells is referred to as "adaptation," and ATM standards have defined five different adaptation layers. ATM Adaptation Layer 1 (AAL1), which is defined in ITU-T Recommendation I.363, allows one to map user data from a constant bit rate source into cells at the source edge of a connection path and reproduce the constant bit rate stream at the destination edge of the connection path. AAL1 provides for the segmentation of the user data into the ATM cells and reassembly of the user data from the cells, the handling of cell delay variation and lost and misinserted cells, the recovery at the receiver of the source clock frequency and the source data structure (i.e., byte and other framing boundaries of the data), and so forth. A Circuit Emulation Service Interoperability Specification (ATM Forum, af-saa-0032.000, September 1995) specifies the ATM Forum's interoperability agreements for supporting constant bit rate traffic over ATM networks. Using the circuit emulation services, standard constant bit rate circuits, such as 64 kb/s PCM channels over a T1 carrier system, can be connected across an ATM network using AAL1.
There are two types of ATM network connections between subscribers, namely, a permanent virtual connection (PVC) and a switched virtual connection (SVC). The PVC and the SVC are distinguished by the manner in which they are established. The PVC is independently established by the network administrator, i.e., established without reference to a particular call, and is the simplest to implement. The SVC is established on demand by the exchange of signaling messages between a caller and the network, and involves the use of both user-access signaling protocols and intra-network signaling protocols.
In a network that uses PVCs, that is, a network that uses pre-established communications paths between users, user data is sent over the network by selecting the appropriate pre-established path based on the destination of the data, and relaying the data in cells through the various switches on the path. The switches relay the cells using the connection path information that was provided by the network administrator when the path was independently established. There is thus no need for a connection establishment protocol between the user and the network, and/or between the network switches. This makes the ATM networks that use PVC's easy to implement. However, such networks cannot dynamically respond to changes in the traffic demands of the users.
With SVCs, the network dynamically establishes and terminates connections in a manner that is analogous to the traditional voice telephone networks discussed above. A network that includes SVCs must support a user connection signaling interface protocol, to exchange connection establishment information such as that defined in the ATM ITU-T User Network Signaling Interface Specification Q.2931. Further, the network must support the intra-network communications that are necessary to establish the connections through the network switches. Thus, a network that uses SVCs is more complicated to implement than one that uses PVCs. Indeed, today most ATM networks use only PVCs.
A PVC may be used to connect constant bit rate circuits through the ATM network. While simple, this provides the equivalent of a leased-line circuit, which cannot be dynamically established to respond to traffic demand changes in the network. Alternatively, an SVC may be used to connect the constant bit rate circuits through the ATM network. Such a connection can be dynamically routed when it is established, and thus, responds to changes in the traffic demand. This is achieved, however, at the expense of an increase in the complexity and the cost of the ATM network.